This invention relates to terrestrial optical and radio frequency (RF) data communication systems. More particularly, the present invention relates to a new and improved method and apparatus for communicating data through a communication link having both a free-space optical path and a parallel wireless RF path. The data is transmitted over the higher capacity optical path when favorable free-space atmospheric conditions prevail, and the data is transmitted over the RF path when free-space atmospheric conditions have degraded the effectiveness of the optical path to the point that it is more efficient to transmit the data over the RF path. Control and status information is preferably transmitted over the more reliable RF path in either circumstance.
The communications industry requires not only high-speed data communication but also reliability in those communications. In terrestrial communications, some of the most common communication links are wire line, radio frequency (RF), fiber optic and free-space optical. Each of these communication links has different relative different strengths, weaknesses and tradeoffs in speed and reliability. Typically, the optical systems have higher communication data rates, speeds or bandwidths, and the wire line, RF and fiber optic links have greater reliability.
Although fiber optic links are capable of both high speed and high reliability, they have the disadvantage of requiring a physical installation of a fiber optic cable as the communication path or media between communication points. A wire line link also requires the physical installation of wires or cables as the communication path or media. In many situations, however, it is impractical, uneconomical or impossible to install physical cable media between communication points. In these situations, the wireless RF and/or free-space optical links must be used.
Free-space optical links transmit a light or laser beam through the atmosphere between an optical transmitter and an optical receiver. The aforementioned US patent application describes an optical transceiver within a mixed fiber and free-space optical communication system. Free-space optical communication systems require a clear line-of-sight path between communication points, since the light or laser beam projects in a straight line between the communication points. The optical beam is subject to degradation by smoke, dust, fog, rain, snow and any other particles in the atmosphere between the communication points. These particles and substances refract or block light beam to the degree that it is not reliably received at the receiving communication point. At times, atmospheric conditions can so severely degrade quality of the light beam between the communication points that the free-space optics can fail to work altogether or that the communication rate is diminished to an unacceptable level.
Wireless RF communication links involve broadcasting an RF signal carrying the communication data between the communication points. Although the typical RF broadcast is capable of transmitting data at a slower rate than an optical signal, the broadcast RF signal is usually more dependable. Broadcast RF signals are not subject to the same degradations from atmospheric conditions that cause free-space optical transmissions to suffer. Although some RF systems, such as microwave systems, do require an unobstructed line-of-sight transmission path, particles and substances in the air do not cause substantial RF signal degradation. Thus, RF communications can operate reliably under conditions where free-space optical transmissions can not operate reliably, thereby providing a greater assurance of accurate and effective data transmission although at a somewhat lesser data transfer rate.
It is with respect to these and other considerations, that the present invention has evolved.
The present invention involves a hybrid wireless optical and radio frequency (RF) communication link or system. Optical transceivers at opposite ends of the link provide an optical path for the primary communication of the data, and RF transceivers primarily provide a communication path for control and status information between the optical and RF transceivers. Under atmospheric conditions that cause the optical communication of data to degrade severely or to fail altogether, data communication is automatically switched to the RF path. Although the overall data communication speed may be reduced when the data is transmitted the RF path, a communication link is maintained under all conditions, rather than suspending data communication during atmospheric conditions which are adverse to optical data communication.
The presence of the RF path between the RF transceivers provides for highly reliable communication of the control and status information, regardless of whether the data is communicated over the optical or RF path. Thus, it is possible to pass control and status information to better control the optical transceivers and their optical signal transmissions, even when the optical link is not operating optimally due to deteriorated free-space atmospheric conditions.
These and other improvements are achieved in an improved method for communicating data in a communication link extending across a terrestrial free-space region between two stations at ends of the link. The method involves communicating the data in an optical signal transmitted through a free-space optical path between the two stations, and communicating the data in a radio frequency (RF) signal transmitted through a free-space RF path between the two stations when the data is not transmitted in the optical signal through the optical path. The optical link is used to transmit the data whenever there is a benefit to using the optical path, and the RF link is used whenever atmospheric conditions in the optical path cause of the optical path to fail or degrade the transmission of the optical signals. A failure or degradation of the optical path is recognized by a failed reception of a transmitted optical signal or by the reception of an optical signal which has been degraded to the point that it is difficult to reliably distinguish the information contained in the optical signal. Control and status information is transferred between the optical transceivers over the RF path to communicate that the optical signal has failed or has degraded. Even while the RF link is transmitting the data, it is preferable to continue attempted transmission of optical signals between the communication points to determine when to reestablish the optical path for the transmission of data with the optical signals can be reliably communicated. The two stations preferably alternately generate and transmit the control and status information and send it to the other station. The control and status information includes information which indicates the amount by which one station should change its optical transmitting power according to the other station""s assessment of the received power, thereby maintaining effective optical communication without oscillation of the power levels of the two stations.
The previously mentioned and other improvements are also achieved in a hybrid wireless optical and radio frequency (RF) communication link for communicating data between first and second stations, where the first and second stations receive and deliver the data through respective first and second input/output (I/O) signal paths. The hybrid communication link includes a free-space optical link portion comprising a first optical transceiver at the first station and a second optical transceiver at the second station for transmitting and receiving an optical signal therebetween containing the data. The hybrid communication link also includes a free-space RF link portion in parallel with the optical link portion and comprising a first RF transceiver at the first station and a second RF transceiver at the second station for transmitting and receiving an RF signal therebetween containing the data and control and status information for controlling the operation of the optical and RF transceivers. The control and status information controls the functionality of the optical transceivers, without diminishing the capacity or bandwidth of the optical transceivers for transmitting and receiving the data contained in the optical signal. In addition, when the optical path fails or degrades because of atmospheric influences, the data is routed for transmission by the RF transceivers over the RF path. Even though the data transferring capability of the RF path is less than that of the optical path, data may still be transferred under conditions where data transfer would be prohibited by a failed or degraded optical path.
The hybrid communication link receives the data to be transmitted over the optical and RF paths from an input/output (I/O) signal path, and the hybrid link transfers the data it receives from the optical and RF paths onto the I/O signal path. A switch within the station at the end communication links routes the data between the optical link and the I/O signal path in an active mode of operation and routes the data between the RF link and the I/O signal path in a standby mode of operation. A transmission status signal is generated as a part of the control and status information, and transmission status signal indicates whether the optical link can effectively transmit data. The switch responds to the transmission status signal to establish either the active or standby modes of operation. An absence of the optical signal in the optical path is also recognized and causes a switch from the active to the standby mode of operation.
A more complete appreciation of the present invention and its scope, and the manner in which it achieves the above noted improvements, can be obtained by reference to the following detailed description of presently preferred embodiments of the invention taken in connection with the accompanying drawings, which are briefly summarized below, and the appended claims.